Coulomb blockade and insulator-to-metal quantum phase transition

We analyze an interplay between Coulomb blockade and quantum fluctuations in a coherent conductor (with dimensionless conductance g larger than unity) 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 $h/e^2\approx 25.8$ 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.