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.