  We take a negative delta function impurity in one arm of a quasi one
dimensional Aharonov-Bohm ring and demonstrate abrupt phase changes across a
quasi bound state of the negative delta function potential. We give a new
mechanism for conductance oscillations with the strength of the negative delta
potential. We also show that coupling to evanescent modes can result in a
$hc/2e$ flux periodicity at certain energies. These observations were made in a
recent ingenious experiment by D. Mailly et al [1].
