  We describe a new regime of magnetotransport in two dimensional electron
systems in the presence of a narrow potential barrier imposed by external
gates. In such systems, the Landau level states, confined to the barrier region
in strong magnetic fields, undergo a deconfinement transition as the field is
lowered. We present transport measurements showing Shubnikov-de Haas (SdH)
oscillations which, in the unipolar regime, abruptly disappear when the
strength of the magnetic field is reduced below a certain critical value. This
behavior is explained by a semiclassical analysis of the transformation of
closed cyclotron orbits into open, deconfined trajectories. Comparison to
SdH-type resonances in the local density of states is presented.
