The aim of this contribution is a better understanding of the conductance changes in nanoslit integrated in microchip device. To this goal voltammetry experiments and modelling were conducted in a hybrid micro nano microchannel (MNM) by taking into account the nanoslit length, the geometry factor influence between the connected microchannels with the main nanoslit, and the background electrolyte concentration. Three models are discussed: (i) a simplified nanochannel conductance model, (ii) a nanochannel conductance model including the role of proton and bicarbonate ionic concentrations on the surface charge state on glass, (iii) a full MNM conductance model including surface and bulk conductance taking into account all the geometric factors for each interface through the MNM device. An eliminating procedure was proposed by subtracting the external microchannels conductances which highlights a net observation of the inverse Duhkin number prediction depending to the nanoslit length. A micromolar concentration zone (from 0.1 µM to 10 µM) is observed where the inverse Duhkin number reaches minimum values around 0.06 and 0.012 for 100 µm-length and 500 µm-length nanoslit, respectively. This latter is in agreement with electro-preconcentration protocol of highly diluted analytes in MNM devices where the nanochannel length plays a preeminent role for maximizing enrichment factor.