  A pair of Dirac points (analogous to a vortex-antivortex pair) associated
with opposite topological numbers (with $\pm\pi$ Berry phases) can be merged
together through parameter tuning and annihilated to gap the Dirac spectrum,
offering a canonical example of a topological phase transition. Here, we report
transport studies on thin films of BiSbTeSe$_2$ (BSTS), which is a 3D TI that
hosts spin-helical gapless (semi-metallic) Dirac fermion surface states (SS)
for sufficiently thick samples, with an observed resistivity close to $h/4e^2$
at the charge neutral point. When the sample thickness is reduced to $\sim$10
nm thick, the Dirac cones from the top and bottom surfaces can hybridize
(analogous to a "merging" in the real space) and become gapped to give a
trivial insulator. Furthermore, we observe that an in-plane magnetic field can
drive the system again towards a metallic behavior, with a prominent negative
magnetoresistance (MR, up to $\sim$$-$95\%) and a temperature-insensitive
resistivity close to $h/2e^2$ at the charge neutral point. The observation is
interpreted in terms of a predicted effect of an in-plane magnetic field to
reduce the hybridization gap (which, if small enough, may be smeared by
disorder and a metallic behavior). A sufficiently strong magnetic field is
predicted to restore and split again the Dirac points in the momentum space,
inducing a distinct 2D topological semimetal (TSM) phase with 2 single-fold
Dirac cones of opposite spin-momentum windings.
