  The effect of 50\% Cu doping at the Au site in the topological Dirac
semimetal CaAuAs is investigated through electronic band structure
calculations, electrical resistivity, and magnetotransport measurements.
Electronic structure calculations a suggest broken-symmetry-driven topological
phase transition from the Dirac to triple-point state in CaAuAs via alloy
engineering. The electrical resistivity of both the CaAuAs and
CaAu$_{0.5}$Cu$_{0.5}$As compounds shows metallic behavior. Nonsaturating
quasilinear magnetoresistance (MR) behavior is observed in CaAuAs. On the other
hand, MR of the doped compound shows a pronounced cusplike feature in the
low-field regime. Such behavior of MR in CaAu$_{0.5}$Cu$_{0.5}$As is attributed
to the weak antilocalization (WAL) effect. The WAL effect is analyzed using
different theoretical models, including the semiclassical $\sim\sqrt{B}$ one
which accounts for the three-dimensional WAL and modified Hikami-Larkin-Nagaoka
model. Strong WAL effect is also observed in the longitudinal MR, which is well
described by the generalized Altshuler-Aronov model. Our study suggests that
the WAL effect originates from weak disorder and the spin-orbit coupled bulk
state. Interestingly, we have also observed the signature of chiral anomaly in
longitudinal MR, when both current and field are applied along the $c$ axis.
The Hall resistivity measurements indicate that the charge conduction mechanism
in these compounds is dominated by the holes with a concentration
$\sim$10$^{20}$ cm$^{-3}$ and mobility $\sim 10^2$ cm$^2$ V$^{-1}$ S$^{-1}$.
