  An apparent h/fe Aharonov-Bohm flux period, where f is an integer, has been
reported in coherent quantum Hall devices. Such sub-period is not expected for
non-interacting electrons and thus is thought to result from interelectron
Coulomb interaction. Here we report experiments in a Fabry-Perot interferometer
comprised of two wide constrictions enclosing an electron island. By carefully
tuning the constriction front gates, we find a regime where interference
oscillations with period h/2e persist throughout the transition between the
integer quantum Hall plateaus 2 and 3, including half-filling. In a large
quantum Hall sample, a transition between integer plateaus occurs near
half-filling, where the bulk of the sample becomes delocalized and thus
dissipative bulk current flows between the counterpropagating edges
("backscattering"). In a quantum Hall constriction, where conductance is due to
electron tunneling, a transition between forward- and back-scattering is
expected near the half-filling. In our experiment, neither period nor amplitude
of the oscillations show a discontinuity at half-filling, indicating that only
one interference path exists throughout the transition. We also present
experiments and an analysis of the front-gate dependence of the phase of the
oscillations. The results point to a single physical mechanism of the observed
conductance oscillations: Aharonov-Bohm interference of interacting electrons
in quantum Hall regime.
