  At low temperatures, quantum corrections, originating from the interference
of the many paths an electron may take between two points, tend to dominate the
transport properties of two-dimensional conductors. These quantum corrections
increase the resistivity in systems such as two-dimensional electron gases
(2DEGs) without spin-orbit coupling (SOC), a phenomenon called weak
localisation. Including symmetry-breaking SOC leads to a change from weak
localisation (WL) to weak anti-localisation (WAL) of the electronic states,
i.e. a WL-to-WAL transition. Here, we revisit the Cooperon, the propagator
encoding quantum corrections, within the context of ultra-clean graphene-based
van der Waals heterostructures with strong symmetry-breaking Bychkov-Rashba SOC
to yield two completely counter-intuitive results. Firstly, we find that
quantum corrections vary non-monotonically with the SOC strength, a clear
indication of non-perturbative physics. Secondly, we observe the exact opposite
of that seen in 2DEGs with strong SOC: a WAL-to-WL transition. This dramatic
reversal is driven by mode entanglement of the pseudospin and spin degrees of
freedom describing graphene's electronic states. We obtain these results by
constructing a non-perturbative treatment of the Cooperon, and observe distinct
features in the SOC dependence of the quantum corrections to the electrical
conductivity that would otherwise be missed by standard perturbative
approaches.
