  We show that a thin film of Weyl or Dirac semimetal with a strong in-plane
magnetic field becomes a novel two-dimensional Fermi liquid with interesting
properties. The Fermi surface in this system is strongly anisotropic, which
originates from a combination of chiral bulk channels and the Fermi arcs. The
area enclosed by the Fermi surface depends strongly on the in-plane magnetic
field component parallel to the Weyl/Dirac node splitting, which leads to
unusual behavior in quantum oscillations when the magnetic field is tilted out
of the plane. We estimate the oscillation frequencies and the regimes where
such effects could be seen in Cd$_3$As$_2$, Na$_3$Bi, and TaAs.
