  Van der Waals materials and their heterostructures offer a versatile platform
for studying a variety of quantum transport phenomena due to their unique
crystalline properties and the exceptional ability in tuning their electronic
spectrum. However, most experiments are limited to devices that have lateral
dimensions of only a few micrometres. Here, we perform magnetotransport
measurements on graphene/hexagonal boron-nitride Hall bars and show that wider
devices reveal additional quantum effects. In devices wider than ten
micrometres we observe distinct magnetoresistance oscillations that are caused
by resonant scattering of Landau-quantised Dirac electrons by acoustic phonons
in graphene. The study allows us to accurately determine graphene's low energy
phonon dispersion curves and shows that transverse acoustic modes cause most of
phonon scattering. Our work highlights the crucial importance of device width
when probing quantum effects and also demonstrates a precise, spectroscopic
method for studying electron-phonon interactions in van der Waals
heterostructures.
