  We study a simple model of dephasing of Aharonov-Bohm oscillations in the
transmission of an electron across a mesoscopic ring. A magnetic impurity in
one of the arms of the ring couples to the electron spin via an exchange
interaction. This interaction leads to spin flip scattering and induces
dephasing via entanglement. This is akin to the models evoked earlier to
explain destruction of interference due to which-path information in
double-slit experiments. Total transmission is found to be symmetric under flux
reversal but not the spin polarization.
