  We demonstrate that a field effect transistor (FET) made of few layer black
phosphorus (BP) encapsulated in hexagonal boron nitride (h-BN) in vacuum,
exhibts the room temperature hole mobility of 5200 $cm^2/Vs$ being limited just
by the phonon scattering. At cryogenic tempeature the FET mobility increases up
to 45,000 $cm^2/Vs$, which is eight times higher compared with the mobility
obtained in earlier reports. The unprecedentedly clean h-BN/BP/h-BN
heterostructure exhibits Shubnikov-de Haas oscillations and quantum Hall effect
with Landau level (LL) filling factors down to v=2 in conventional laboratory
magnetic fields. Moreover, carrier density independent effective mass m=0.26
m_0 is measured, and Lande g-factor g=2.47 is reported. Furthermore, an
indication for a distinct hole transport behavior with up and down spin
orientation is found.
