  Strange metals exhibit a variety of anomalous magnetotransport properties,
the most striking of which is a resistivity that increases linearly with
magnetic field $B$ over a broad temperature and field range. The ubiquity of
this behavior across a spectrum of correlated metals - both single- and
multi-band, with either dominant spin and/or charge fluctuations, of varying
levels of disorder or inhomogeneity and in proximity to a quantum critical
point or phase - obligates the search for a fundamental underlying principle
that is independent of the specifics of any material. Strongly anisotropic
(momentum-dependent) scattering can generate $B$-linear magnetoresistance but
only at intermediate field strengths. At high enough fields, the
magnetoresistance must eventually saturate. Here, we consider the ultimate
limit of such anisotropy, a region or regions on the Fermi surface that impede
all orbital (cyclotron) motion through them, but whose imposition can be
modelled nonetheless through a modified Boltzmann theoretical treatment.
Application of the proposed theorem suggests that the realization of
quadratic-to-linear magnetoresistance requires the presence of a bounded sector
on the Fermi surface possibly separating two distinct types of carriers. While
this bounded sector may have different origins or manifestations, we expect its
existence to account for the anomalous magnetotransport found in a wide range
of correlated materials.
