  We study a theoretical model of virtual scanning tunneling microscopy (VSTM),
a proposed application of interlayer tunneling in a bilayer system to locally
probe a two-dimensional electron system (2DES) in a semiconductor
heterostructure. We consider tunneling for the case where transport in the
2DESs is ballistic, and show that the zero-bias anomaly is suppressed by
extremely efficient screening. Since such an anomaly would complicate the
interpretation of data from a VSTM, this result is encouraging for efforts to
implement such a microscopy technique.
