  We consider a $\mathbb{R}$-extension of one dimensional uniformly expanding
open dynamical systems and prove a new explicit estimate for the asymptotic
spectral gap. To get these results, we use a new application of a "global
normal form" for the dynamical system, a "semiclassical expression beyond the
Ehrenfest time" that expresses the transfer operator at large time as a sum
over rank one operators (each is associated to one orbit). In this paper we
establish the validity of the so-called "diagonal approximation" up to twice
the local Ehrenfest time.
