  Chiral anomaly induced negative magnetoresistance (NMR) has been widely used
as a critical transport evidence on the existence of Weyl fermions in
topological semimetals. In this mini review, we discuss the general observation
of the NMR phenomena in non-centrosymmetric NbP and NbAs. We show that NMR can
be contributed by intrinsic chiral anomaly of Weyl fermions and/or extrinsic
effects, such as superimposition of Hall signals, field-dependent inhomogeneous
current flow in the bulk, i.e. current jetting, and weak localization (WL) of
coexistent trivial carriers. Such WL controlled NMR is heavily dependent on
sample quality, and is characterized by pronounced crossover from positive to
negative MR growth at elevated temperatures, as a result of the competition
between the phase coherence time and the spin-orbital scattering constant of
the bulk trivial pockets. Thus, the correlation of NMR and chiral anomaly needs
to be scrutinized, without the support of other complimentary techniques. Due
to the lifting of spin degeneracy, the spin orientations of Weyl fermions are
either parallel or antiparallel to the momentum, a unique physical property
known as helicity. The conservation of helicity provides strong protection for
the transport of Weyl fermions, which can only be effectively scattered by
magnetic impurities. Chemical doping of magnetic and non-magnetic impurities
are thus more convincing in probing the existence of Weyl fermions than the NMR
method.
