  We report a topological phase transition in quantum-confined cadmium arsenide
(Cd3As2) thin films under an in-plane Zeeman field when the Fermi level is
tuned into the topological gap via an electric field. Symmetry considerations
in this case predict the appearance of a two-dimensional Weyl semimetal (2D
WSM), with a pair of Weyl nodes of opposite chirality at charge neutrality that
are protected by space-time inversion (C2T) symmetry. We show that the 2D WSM
phase displays unique transport signatures, including saturated resistivities
on the order of h/e^2 that persist over a range of in-plane magnetic fields.
Moreover, applying a small out-of-plane magnetic field, while keeping the
in-plane field within the stability range of the 2D WSM phase, gives rise to a
well-developed odd integer quantum Hall effect, characteristic of degenerate,
massive Weyl fermions. A minimal four-band k.p model of Cd3As2, which
incorporates first-principles effective g factors, qualitatively explains our
findings.
