Finite-Temperature Transport Properties of Normal Fermi Systems. I. The General Transport Equation

We express the response of a normal Fermi system to a weak external charge in terms of a one-particle Wigner distribution function which obeys a generalized transport equation valid at all temperatures. The derivation of this transport equation is closely related to the method developed by Prigogine and co-workers in nonequilibrium situations, both classical and quantum; it is based on a reclassification of the perturbation expansion of the autocorrelation formula expressing the response. The proof is greatly facilitated by the use of a suitable diagram technique which describes both the dynamical processes in the system as well as the effect of the interactions on equilibrium properties; full advantage is taken of the contraction theorem for averages over the unperturbed equilibrium ensemble.