  Charged particles in static electric and magnetic fields have Landau levels
and tunneling states from the vacuum. Using the instanton method of Phys. Rev.
D 65, 105002 (2002), we obtain the formulae for the pair-production rate in
spinor and scalar QED, which sum over all Landau levels and recover exactly the
well-known results. The pair-production rates are calculated for an electric
field of finite extent, with a constant magnetic field also present, and are
shown to have boundary effects.
