Electron-hole coherence and charging effects in ultrasmall metallic grains

We consider a model for electron tunneling between a pair of ultrasmall metallic grains. Under appropriate circumstances, nonequilibrium final state effects can strongly enhance tunneling and produce electron-hole coherence between the grains. The model displays a quantum phase transition between a Coulomb blockaded state to a coherent state exhibiting sub-Ohmic tunneling conductance. The critical state of the junction exhibits a temperature independent resistance of order h/e(2). Finally we discuss the similarities between the quantum transition in our model and the metal-insulator transition in granular wires observed by Herzog et al.