  We calculate the junction resistance for a metal-barrier-metal device with
the barrier tuned to lie just on the insulating side of the metal-insulator
transition. We find that the crossover from tunneling behavior in thin barriers
at low temperature to incoherent transport in thick barriers at higher
temperature is governed by a generalized Thouless energy. The crossover
temperature can be estimated from the low temperature resistance of the device
and the bulk density of states of the barrier.
