We demonstrate that the polarization of the optical modes in wirelike micro
cavities formed by lithography can be controlled by an external magnetic fi
eld, At zero field, these modes are split into two linearly polarized compo
nents by the resonator geometry with a splitting which depends on the wire
width and the mode index. A magnetic field causes these modes to become cir
cularly polarized and their splittings to increase. These effects are shown
to result from magnetic-field dependence of the dielectric function.