Stability of A-B phase boundary in a constriction

We report on a Ginzburg-Landau calculation of the stability of a boundary between A and B phases of superfluid 3He in a two-dimensional constriction. In the macroscopic limit the stability follows a well-known relation, which depends on the surface tension σAB of the A-B boundary. In the narrow-constriction limit the surface tension is not well defined, but the interface is always stable, and a weak link between the A and B phases is obtained