Justification of the Use of Perturbation Theory in Metallic Conductivity

In metals at high temperatures the time τ spent by an electron between two consecutive scatterings is smaller than the duration of the collision tc, if one defines tc as that time after which one can be sure that the interaction with the scattering centre (or lattice wave) is less than kT. Hence the customary theory of conductivity is not justified a priori because it considers the scatterings to be independent. Yet its results are confirmed by experiment. In this paper the correction terms for scattering by two centres are calculated. They are shown to be negligible for a Fermi gas.