Kinetic coefficients in a time-dependent Green's function formalism at finite temperature

We discuss microscopic foundations of dissipation arising in a model Fermi or Bose system with weak local interaction. We consider the dynamics of equilibrium fluctuations in the Keldysh-Schwinger formalism and discuss the relation between dissipation and pinch singularities of perturbation theory diagrams. Using the Dyson equation, we define and calculate the dissipation parameter in the two-loop approximation. We show that this parameter is analogous to Onsager's kinetic coefficient and is associated with decay in the quasiparticle spectrum.