Materials science has found new and exciting tools following the development of the scanning tunneling microscope (STM), after which other instruments based on similar principles have been developed [1]. The possibility of studying the processes that occur at the atomic level when two materials are brought into contact is very promising for understanding problems of enormous basic and technological interest such as contact formation, adhesion, friction, wear, fracture dynamics, materials hardness, surface deformations, and many others. Macroscopic contact between two bodies is the result of contact between microscopic asperities, and consequently the nanomechanical properties of these asperities will be responsible for the macroscopic behavior. Recently several groups have reported molecular dynamics calculations [2 ‐4] which provide deep insight on several aspects of the formation, plastic deformation, and fracture of the connective neck formed when a clean metal asperity interacts with a clean metal surface. In this Letter, we present a detailed experimental study of the nanomechanical behavior of the connective necks formed between a tip and a substrate of the same metal (Au) by cohesive bonding after contact. The radius of these necks ranges from 1 to 8 nm. This experimental approach is quite different from that of other nanomechanical studies [5,6] in which a hard tip of about 100 nm radius indents a softer material substrate. In previous work we studied the plastic deformation of Pb connective necks [7] using an STM. The stepwise variation of the conductance as the tip was moved perpendicularly to the substrate was attributed to the alternation of elastic and yielding stages during the deformation process. Now we have added force measuring capability to make possible a complete characterization of metal connective necks from a nanomechanical point of view. To our knowledge this is the first quantitative experimental report to follow the plastic deformation process with such minute detail. The experimental setup is shown in the inset of Fig. 1. The force exerted by the Au tip on the Au substrate is obtained by measuring the deflection of a stiff phosphorous bronze cantilever (elastic constant 705 Nym) on which the substrate is mounted, with an auxiliary tunneling tip. This auxiliary tunneling tip works in the constant current mode, that is, at constant tunneling gap distance, and consequently forces exerted by this tip on the cantilever are constant and need not be taken into account [8]. In addition to the tunneling voltage applied between the auxiliary STM tip and the cantilever, we apply a voltage difference of 10 mV [9] between the Au tip and Au substrate in order to measure the neck conductance. This is the same setup we used in a previous work [10], but with improved resolution. The experiment is conducted in vacuum at liquid helium temperature (4.2 K). Under this condition thermal drift and creep effects in the piezoelectric transducers [11] are negligible, and capillary forces are completely avoided, resulting in a very good reproducibility of the measurements.