  The family of binary Lanthanum monopnictides, LaBi and LaSb, have attracted a
great deal of attention as they display an unusual extreme magnetoresistance
(XMR) that is not well understood. Two classes of explanations have been raised
for this: the presence of non-trivial topology, and the compensation between
electron and hole densities. Here, by synthesizing a new member of the family,
LaAs, and performing transport measurements, Angle Resolved Photoemission
Spectroscopy (ARPES), and Density Functional Theory (DFT) calculations, we show
that (a) LaAs retains all qualitative features characteristic of the XMR effect
but with a siginificant reduction in magnitude compared to LaSb and LaBi, (b)
the absence of a band inversion or a Dirac cone in LaAs indicates that topology
is insignificant to XMR, (c) the equal number of electron and hole carriers
indicates that compensation is necessary for XMR but does not explain its
magnitude, and (d) the ratio of electron and hole mobilities is much different
in LaAs compared to LaSb and LaBi. We argue that the compensation is
responsible for the XMR profile and the mobility mismatch constrains the
magnitude of XMR.
