  In a previous paper [1], it was shown that the worldline expression for the
nonperturbative imaginary part of the QED effective action can be approximated
by the contribution of a special closed classical path in Euclidean spacetime,
known as a worldline instanton. Here we extend this formalism to compute also
the prefactor arising from quantum fluctuations about this classical closed
path. We present a direct numerical approach for determining this prefactor,
and we find a simple explicit formula for the prefactor in the cases where the
inhomogeneous electric field is a function of just one spacetime coordinate. We
find excellent agreement between our semiclassical approximation, conventional
WKB, and recent numerical results using numerical worldline loops.
