Isothermal resistivity for interacting charged particles in contact with a bath

To calculate the stationary resistivity, the role of the coupling to a bath is pointed out. In the case of two kinds of interacting charged particles, the coupling to a bath is taken into account by relaxation terms in the equation of motion of the density operator so that the correct balance equations for the impuls transfer are obtained. Higher orders of a perturbation expansion of the resistivity with respect to the coupling constant are given. For special cases the Matthiesen rule is derived, and deviations from the Matthiesen rule are discussed.