  An interplay between charge discreteness, coherent scattering and Coulomb
interaction yields nontrivial effects in quantum transport. We derive a real
time effective action and an equivalent quantum Langevin equation for an
arbitrary coherent scatterer and evaluate its current-voltage characteristics
in the presence of interactions. Within our model, at large conductances $G_0$
and low $T$ (but outside the instanton-dominated regime) the interaction
correction to $G_0$ saturates and causes conductance suppression by a universal
factor which depends only on the type of the conductor.
