  We derive a formula for the electrical conductivity of solids that includes
relaxation, dissipation, and quantum coherence. The derivation is based on the
Kubo formula, with a Mori memory function approach to include dissipation
effects at all orders in the relaxation interaction. It offers a practical
method to evaluate the conductivity with electronic-structure codes and avoids
the complications and limitations of the Kubo formula in the thermodynamic
limit. The derivation of our formula provides a method applicable to other
transport coefficients and correlation functions.
