  We investigate the decay of quantum electrodynamical (QED) vacuum in
arbitrary space-dependent electric fields. In particular, we analyze the
resonance peaks of the positron emission spectrum for the external fields with
subcycle structure. For this, we study the transmission probability in the
framework of scattering approach to vacuum pair production. In
under-the-barrier scattering regime, we show that the width of a transmission
resonance can be enhanced when the effective scattering potential contains
multiple wells. Such a broadening in the resonance width corresponds to a
decrease in the tunneling time. This may be relevant for observing the vacuum
decay at shorter timescales before the external field is adiabatically turned
off. In above-the-barrier scattering regime, we give a set of coupled
differential equations for the numerical computation of the Bogoliubov
coefficients.
