  This paper presents the derivation of Schwinger's gauge invariant result of
$Im \cal{L}_{eff}$ upto one loop approximation, for particle production in an
uniform electric field through the method of complex trajectory WKB
approximation (CWKB). The CWKB proposed by one of the author's \cite{bis:ijtp}
looks upon particle production as due to the motion of a particle in complex
space-time plane, thereby requiring tunneling paths both in space and time.
Recently \cite{srini:iucaa,srini1:iucaa} there have been some efforts to
calculate the reflection and transmission co-efficients for particle production
in uniform electric field that differ from our expressions for the same. In
this paper we clarify the confusion in this regard and establish the
correctness of CWKB.
