  This work was motivated by the quest for observing interference of
fractionally charged quasi particles. Here, we study the behavior of an
electronic Mach-Zehnder interferometer (MZI) at the integer quantum Hall effect
(IQHE) regime at filling factors greater than one. Both the visibility and the
drift velocity were measured, and were found to be highly correlated as
function of filling factor. As the filling factor approached unity, the
visibility quenched, not to recover for filling factors smaller than unity.
Alternatively, the velocity saturated around a minimal value at unity filling
factor. We highlight the significant role interactions between the interfering
edge and the bulk play, as well as that of the defining potential at the edge.
Shot noise measurements suggest that phase-averaging (due to phase
randomization), rather than single particle decoherence, is likely to be the
cause of the dephasing in the fractional regime.
