  We study the quantum interference effect for the single ballistic
Aharonov-Bohm billiard in the presence of a weak magnetic field B. The diagonal
part of the wave-number averaged reflection coefficient $\delta {\cal R}_D$ is
calculated by use of semi-classical scattering theory. In addition to the
appearance of "h/2e oscillation" that are caused by interference between
time-reversed coherent backscattering classical trajectories, B in the
conducting region leads to negative magnetoresistance and dampening of the h/2e
oscillation amplitude. The B dependence of the results reflects the underlying
classical (chaotic and regular) dynamics.
