  Weak localization was observed and determined in a black phosphorus (bP)
field-effect transistor 65 nm thick. The weak localization behaviour was found
to be in excellent agreement with the Hikami-Larkin-Nagaoka model for fields up
to 1~T, from which characteristic scattering lengths could be inferred. The
dephasing length $L_\phi$ was found to increase linearly with increasing hole
density attaining a maximum value of 55 nm at a hole density of approximately
$10^{13} cm^{-2}$ inferred from the Hall effect. The temperature dependence of
$L_\phi$ was also investigated and above 1~K, it was found to decrease weaker
than the $L_\phi \propto T^{-\frac{1}{2}}$ dependence characteristic of
electron-electron scattering in the presence of elastic scattering in two
dimensions. Rather, the observed power law was found to be close to that
observed previously in other quasi-one-dimensional systems such as metallic
nanowires and carbon nanotubes. We attribute our result to the crystal
structure of bP which host a `puckered' honeycomb lattice forming a strongly
anisotropic medium
