  The influence of a magnetic field on the tunneling of an electron out of a
confining plane is studied by a path integral method. We map this 3-d problem
on to a 1-d one, and find that the tunneling is strongly affected by the field.
Without a perpendicular field the tunneling at zero temperature can be
completely suppressed by a large parallel field, but in the small parallel
field and low temperature limit the tunneling rate is finite. An explicit
formula is obtained in this case. A quantitative explanation without fitting
parameter to a recent experiment is provided.
