  We show how to use a worldline-instanton formalism to calculate, to leading
order in the weak-field expansion, the momentum spectrum of nonlinear
Breit-Wheeler pair production in fields that depend on time and one spatial
coordinate. We find a nontrivial dependence on the width, $\lambda$, of the
photon wave packet, and the existence of a critical point $\lambda_c$. For
$\lambda<\lambda_c$ and a field with one peak, the spectrum has one peak where
the electron and positron have the same energy. For $\lambda>\lambda_c$ this
splits into two peaks. We calculate a high-energy ($\Omega\gg1$) expansion,
which to leading order agrees with the results obtained by replacing the
space-time field with a plane wave and using the well-known Volkov solutions.
We also calculate an expansion for $\Omega\sim a_0\gg1$, where the field is
strong enough to significantly bend the trajectories of the fermions despite
$\Omega\gg1$.
