  Two dimensional materials are usually envisioned as flat, truly 2D layers.
However out-of-plane corrugations are inevitably present in these materials. In
this manuscript, we show that graphene flakes encapsulated between insulating
crystals (hBN, WSe2), although having large mobilities, surprisingly contain
out-of-plane corrugations. The height fluctuations of these corrugations are
revealed using weak localization measurements in the presence of a static
in-plane magnetic field. Due to the random out-of-plane corrugations, the
in-plane magnetic field results in a random out-of-plane component to the local
graphene plane, which leads to a substantial decrease of the phase coherence
time. Atomic force microscope measurements also confirm a long range height
modulation present in these crystals. Our results suggest that phase coherent
transport experiments relying on purely in-plane magnetic fields in van der
Waals heterostructures have to be taken with serious care.
