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

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... suppression by a universal factor which depends only on the type of the conductor.