The scattering theory of transport has been shown to provide a rigorous theoretical framework for the description of normal mesoscopic systems. This approach is based on the observation that, as long as dissipative processes in the active region can be neglected, each scattering eigenstate communicates with one and only one reservoir and as such can be assumt:d to be in equilibrium with that reservoir. This allows us to calculate any quantity of interest under non-equilibrium conditions using a simple extension of equilibrium statistical mechanics, without the need to solve complicated kinetic equations. In this review we discuss how the same approach can be applied to mesoscopic systems including superconducting segments and relate this approach to the commonly used Green's function formalism.
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