  It has been shown how on-shell forward scattering amplitudes (the ``Barton
expansion'') and quantum mechanical path integral (QMPI) can both be used to
compute temperature dependent effects in thermal field theory. We demonstrate
the equivalence of these two approaches and then apply the QMPI to compute the
high temperature expansion for the four-point function in QED, obtaining
results consistent with those previously obtained from the Barton expansion.
