  We derive a closed equation of motion for the one particle density matrix of
a quantum system coupled to multiple baths using the Redfield master equation
combined with a mean-field approximation. The steady-state solution may be
found analytically with perturbation theory. Application of the method to a
one-dimensional non-interacting quantum wire yields an expression for the
current that reproduces the celebrated Landauer's formula. Nonlinear
rectification is found for the case of a mesoscopic three-dimensional
semiconductor p-n junction. The results are in good agreement with numerical
simulations obtained using non-equilibrium Green's functions, supporting the
validity of the Redfield equations for the description of transport.
