  Non-centrosymmetric transition metal monopnictides, including TaAs, TaP,
NbAs, and NbP, are emergent topological Weyl semimetals (WSMs) hosting exotic
relativistic Weyl fermions. In this letter, we elucidate the physical origin of
the unprecedented charge carrier mobility of NbP, which can reach
$1\times10^{7}$ cm $^{2}$V$^{-1}$s$^{-1}$ at 1.5 K. Angle- and
temperature-dependent quantum oscillations, supported by density function
theory calculations, reveal that NbP has the coexistence of p- and n-type WSM
pockets in the $k_{z}$=1.16$\pi$/c plane (W1-WSM) and in the $k_{z}$=0 plane
near the high symmetry points $\Sigma$ (W2-WSM), respectively. Uniquely, each
W2-WSM pocket forms a large dumbbell-shaped Fermi surface (FS) enclosing two
neighboring Weyl nodes with the opposite chirality. The magneto-transport in
NbP is dominated by these highly anisotropic W2-WSM pockets, in which Weyl
fermions are well protected from defect backscattering by real spin
conservation associated to the chiral nodes. However, with a minimal doping of
$\sim$1\% Cr, the mobility of NbP is degraded by more than two order of
magnitude, due to the invalid of helicity protection to magnetic impurities.
Helicity protected Weyl fermion transport is also manifested in chiral anomaly
induced negative magnetoresistance, controlled by the W1-WSM states. In the
quantum regime below 10 K, the intervalley scattering time by impurities
becomes a large constant, producing the sharp and nearly identical conductivity
enhancement at low magnetic field.
