  Longitudinal magnetoresistance (LMR) refers to the change in resistance due
to a magnetic field when the current and the magnetic field are parallel to
each other. For this to be nonzero in weak magnetic fields, kinetic theory
stipulates that the electronic dispersion must satisfy certain conditions: it
should either be sufficiently anisotropic or have topological features. The
former results in a positive LMR, while the latter results in a negative LMR.
Here, I propose a different mechanism that leads to LMR in any dispersion
without a need to satisfy the above requirements. The mechanism is quantum in
origin but is applicable in the said regime. It arises due to the change in the
density of states with the magnetic field and is not kinetic in origin.
Remarkably, LMR is found to be negative even if the dispersion is
nontopological, provided it is nonparabolic. An analytical expression is
derived for this contribution to LMR. It is found to depend on the orbital
magnetic susceptibility. The analytical findings are confirmed by numerical
calculations.
