  The remarkably large persistent currents that are observed in disordered
micron-scale gold rings at low temperatures have recently been explained in a
theory of non-interacting electrons scattered by crystal grain boundaries. The
present article examines the possibility that the basic physics underlying this
explanation may also have observable consequences in a different system, a
ballistic two-dimensional semiconductor ring with a potential barrier. It is
predicted, using computer simulations, that such semiconductor rings can
exhibit large persistent currents ~ evF/L (L is the ring circumference),
despite the electron transmission probability through the barrier being small,
in the regime of quantum-mechanical tunneling of electrons through the barrier.
This, like the phenomenon observed in gold rings, is a manifestation of the
fundamental dissimilarity of non-equilibrium transport phenomena and
equilibrium persistent currents.
