  We report experiments on Fabry-Perot electron interferometers in the integer
quantum Hall regime. The GaAs/AlGaAs heterostructure devices consist of two
constrictions defined by etch trenches in 2D electron layer, enclosing an
approximately circular island. The interferometer is formed by
counterpropagating chiral edge channels coupled by tunneling in the two
constrictions. Interference fringes are observed as conductance oscillations,
similar to the Aharonov-Bohm effect. Front gates deposited in etch trenches
allow to fine-tune the device and to change the constriction filling f relative
to the bulk filling. Quantum-coherent conductance oscillations are observed on
the f = 1 - 4 plateaus. On plateau f we observe f conductance oscillations per
fundamental flux period h/e. This is attributed to the dominance of the
electron-electron Coulomb interaction, effectively mixing Landau level
occupation. On the other hand, the back-gate charge period is the same (one
electron) on all plateaus, independent of filling. This is attributed to the
self-consistent electrostatics in the large electron island. We also report
dependence of the oscillation period on front-gate voltage for f = 1, 2 and 4
for three devices. We find a linear dependence, with the slope inversely
proportional to f for f = 1 and 2.
