  We have introduced a controllable nano-scale incursion into a potential
barrier imposed across a two-dimensional electron gas, and report on the
phenomena that we observe as the incursion develops. In the quantum Hall
regime, the conductance of this system displays quantized plateaus, broad
minima and oscillations. We explain these features and their evolution with
electrostatic potential geometry and magnetic field as a progression of current
patterns formed by tunneling between edge and localized states within the
barrier.
