e-h 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, non-equilibrium 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 subohmic tunneling conductance. The critical state of the junction exhibits a temperature independent resistance of order $h/e^2$. Finally we discuss the possible relevance to granular materials and quantum dots. In particular, similarities between the quantum transition in our model and the metal-insulator transition in granular wires observed by Herzog et al. are described in detail.