We investigate the dependence of decoherence on the mode number M in a multiple-mode Aharonov-
Bohm (AB) interferometer. The design of the AB interferometer allows us to precisely determine M by the
additivity rule of ballistic conductors; meanwhile, the decoherence rate is simultaneously deduced by the
variance of the AB oscillation amplitude. The AB amplitude decreases and fluctuates with depopulating
M. Moreover, the normalized amplitude exhibits a maximum at a specific M (∼9). Data analysis reveals
that the charge-fluctuation-induced dephasing, which depends on the geometry and the charge relaxation
resistance of the system, could play an essential role in the decoherence process. Our results suggest that the
phase coherence, in principle, can be optimized using a deliberated design and pave one of the ways toward
the engineering of quantum coherence.
