We analyze an interplay between Coulomb blockade and quantum fluctuations in a coherent conductor (with dimensionless conductance g > ∼ 1) attached to an Ohmic shunt. We demonstrate that at T = 0 the system can be either an insulator or a metal depending on whether its total resistance is larger or smaller than h/e 2 ≈ 25.8 kΩ. In a metallic phase the Coulomb gap is fully suppressed by quantum fluctuations. We briefly discuss possible relation of this effect to recent experiments indicating the presence of a metal-insulator phase transition in 2d disordered systems.It is well known that Coulomb interaction may strongly affect quantum transport of electrons in mesoscopic conductors. For instance, electron tunneling across metallic junctions can be strongly suppressed or even blocked completely at T = 0 due to Coulomb effects [1][2][3]. This Coulomb blockade of tunneling is a direct consequence of the electron charge discreteness, manifestations of which persist even for junctions with resistances well below the quantum resistance unit R Q = h/e 2 ≈ 25.8 kΩ. Recently it was argued [4] that the I − V curve of an arbitrary -albeit relatively short -coherent conductor in the presence of interactions can be expressed in the formwhere 1/R = (2e 2 /h) n T n is the Landauer conductance of a scatterer and T n are the transmissions of its conducting modes. The magnitude of the interaction term in eq. (1) is controlled by the parameterand f (V, T ) is a universal function which depends on R as well as on the external impedance Z S (ω) (e.g. leads) attached to the scatterer. A similar result was also obtained in Ref. [5] in the limit of a single conducting mode, in which case β = 1 − T 1 . The parameter (2) is already well known in the theory of shot noise [6]. Since shot noise and interaction effects in mesoscopic conductors are both the manifestations of the electron charge discreteness, it appears quite natural that the magnitude of both these effects is governed by the same parameter β (2).It is also important that the above effects do not disappear even if the charge quantization is not preserved in a part of the system (e.g. in the leads). Consider an important example of an Ohmic external impedance Z S (ω) ≃ R S and define g S = R Q /R S and g 0 = g + g S . The function f (V, T ) was already evaluated before for the case of tunnel junctions. At not very high energies max(T, eV ) ≪ g 0 E C and for g 0 ≫ 1 one findsi.e. in this regime Coulomb interaction causes a logarithmic correction to the scatterer conductance. This correction is small in the parameter 1/g 0 . Here E C is the effective charging energy which sets the energy scale for the interaction effects in our problem. The result (3) is valid down to energies max(T, eV ) ∼ g 0 E C exp(−g 0 /2). At even lower T and eV energy relaxation effects turn out to be particularly important making the conductance saturate at a universal value [4]This formula recovers complete Coulomb blockade in the limit β → 1 (tunnel junctions), demonstrates the absence of it for ballisti...