Coherent tunneling through localized states

We present results of tunneling experiments performed on Au/InO_x/Pb junctions. At low temperatures, two types of tunneling can be identified in these structures: Single-particle tunneling and pair-tunneling. The former is associated with low-transmission channels while the latter points to the existence of resonances. The contribution of pair-tunneling is shown to be extremely sensitive to the application of a microwave field. It is demonstrated that the effect of the microwaves is distinctly different than that produced by heating and apparently involves transitions within the localized states of the Anderson-insulating InO_x barrier.