  The Mott-Ioffe-Regel limit sets the lower bound of carrier mean free path for
coherent quasiparticle transport. Metallicity beyond this limit is of great
interest because it is often closely related to quantum criticality and
unconventional superconductivity. Progress along this direction mainly focuses
on the strange-metal behaviors originating from the evolution of quasiparticle
scattering rate such as linear-in-temperature resistivity, while the
quasiparticle coherence phenomena in this regime are much less explored due to
the short mean free path at the diffusive bound. Here we report the observation
of quantum oscillations from Landau quantization near the Mott-Ioffe-Regel
limit in CaAs3. Despite the insulator-like temperature dependence of
resistivity, CaAs3 presents giant magnetoresistance and prominent Shubnikov-de
Haas oscillations from Fermi surfaces, indicating highly coherent band
transport. In contrast, the quantum oscillation is absent in the magnetic
torque. The quasiparticle effective mass increases systematically with magnetic
fields, manifesting a much larger value than the expectation given by
magneto-infrared spectroscopy. It suggests a strong many-body renormalization
effect near Fermi surface. We find that these unconventional behaviors may be
explained by the interplay between the mobility edge and the van Hove
singularity, which results in the formation of coherent cyclotron orbits
emerging at the diffusive bound. Our results call for further study on the
electron correlation effect of the van Hove singularity.
