  We analyse when and why unitarity violations might occur in quantum cosmology
restricted to minisuperspace. To this end we discuss in detail backscattering
transitions between expanding and contracting solutions of the Wheeler-DeWitt
equation. We first show that upon neglecting only backscattering, one obtains
an intermediate regime in which matter evolves unitarily but which does not
correspond to any Schr\"odinger equation in a given geometry since
gravitational backreaction effects are taken into account at the quantum level.
We then show that backscattering amplitudes are exponentially smaller than
matter transition amplitudes. Both results follow from an adiabatic treatment
valid for macroscopic universes. To understand how backscattering and the
intermediate regime should be interpreted, we review the problem of electronic
transitions induced by nuclear motion since it is mathematically very similar.
In this problem, transition amplitudes are obtained from the conserved current.
The same applies to quantum cosmology and shows that the unique consistent
interpretation is based on the current when backscattering is neglected. We
then review why, in a relativistic context, backscattering is interpreted as
pair production whereas it is not in the non relativistic case. In each example
the correct interpretation is obtained by coupling the system to an external
quantum device. From the absence of such external systems in cosmology, we
conclude that backscattering does not have a unique consistent interpretation
in quantum cosmology.
