We have measured tunneling rates out of an electron layer trapped
at a liquid-helium—vacuum interface in the presence of a magnetic field.
When the field is transverse to the escape direction we find a striking
suppression of the tunneling rates: a field of 3000 G at 40 mK reduces
the tunneling current by four orders of magnitude. As the temperature increases
the magnetic suppression of tunneling diminishes until it disappears completely
above 250 mK. By contrast parallel fields have no effect on the tunneling
rates.