  We report the magneto-transport properties of CaAl$_4$ single crystals with
$C2/m$ structure at low temperature. CaAl$_4$ exhibits large unsaturated
magnetoresistance $\sim$3000$\%$ at 2.5 K and 14 T. The nonlinear Hall
resistivity is observed, which indicates the multi-band feature. The
first-principles calculations show the electron-hole compensation and the
complex Fermi surface in CaAl$_4$, to which the two-band model with
over-simplified carrier mobility can't completely apply. Evident quantum
oscillations have been observed with B//c and B//ab configurations, from which
the nontrivial Berry phase is extracted by the multi-band Lifshitz-Kosevich
formula fitting. An electron-type quasi-2D Fermi surface is found by the
angle-dependent Shubnikov-de Haas oscillations, de Haas-van Alphen oscillations
and the first-principles calculations. The calculations also elucidate that
CaAl$_4$ owns a Dirac nodal line type band structure around the $\Gamma$ point
in the $Z$-$\Gamma$-$L$ plane, which is protected by the mirror symmetry as
well as the space inversion and time reversal symmetries. Once the spin-orbit
coupling is included, the crossed nodal line opens a negligible gap (less than
3 meV). The open-orbit topology is also found in the electron-type Fermi
surfaces, which is believed to help enhance the magnetoresistance observed.

Materials Science;Mesoscale and Nanoscale Physics