A method is presented to determine the mean free path in individual layers of giant magnetoresistive samples from experimental data. The boundary conditions at the surfaces of the thin films are fixed at certain positions within the stack by the implementation of foreign atoms. The scattering activities of these atoms are analysed by means of the GMR response of specially designed samples. An analytical expression is used to model the variation of the sheet conductance as a function of the thickness of Cu and Co thin sputtered films. The data of Cu are well reproduced by means of a single bulk mean free path, whereas for Co, a spin dependence of the mean free paths is needed.