  We show that the emergence of the axial anomaly is a universal phenomenon for
a generic three dimensional metal in the presence of parallel electric ($E$)
and magnetic ($B$) fields. In contrast to the expectations of the classical
theory of magnetotransport, this intrinsically quantum mechanical phenomenon
gives rise to the longitudinal magnetoresistance for any three dimensional
metal. However, the emergence of the axial anomaly does not guarantee the
existence of negative longitudinal magnetoresistance. We show this through an
explicit calculation of the longitudinal magnetoconductivity in the quantum
limit using the Boltzmann equation, for both short-range neutral and long-range
ionic impurity scattering processes. We demonstrate that the ionic scattering
contributes a large positive magnetoconductivity $\propto B^2$ in the quantum
limit, which can cause a strong negative magnetoresistance for any three
dimensional or quasi-two dimensional metal. In contrast, the finite range
neutral impurities and zero range point impurities can lead to both positive
and negative longitudinal magnetoresistance depending on the underlying band
structure. In the presence of both neutral and ionic impurities, the
longitudinal magnetoresistance of a generic metal in the quantum limit
initially becomes negative, and ultimately becomes positive after passing
through a minimum. We discuss in detail the qualitative agreement between our
theory and recent observations of negative longitudinal magnetoresistance in
Weyl semimetals TaAs and TaP, Dirac semimetals Na$_3$Bi, Bi$_{1-x}$Sb$_x$, and
ZrTe$_5$, and quasi-two dimensional metals PdCoO$_2$,
$\alpha$-(BEDT-TTF)$_2$I$_3$ which do not possess any bulk three dimensional
Dirac or Weyl quasiparticles.
