  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.
