  Dirac states hosted by Sb/Bi square nets are known to exist in the layered
antiferromagnetic AMnX$_2$ (A = Ca/Sr/Ba/Eu/Yb, X=Sb/Bi) material family the
space group to be P4/nmm or I4/mmm. In this paper, we present a comprehensive
study of quantum transport behaviors, angle-resolved photoemission spectroscopy
(ARPES) and first-principles calculations on SrZnSb2, a nonmagnetic analogue to
AMnX2, which crystallizes in the pnma space group with distorted square nets.
From the quantum oscillation measurements up to 35 T, three major frequencies
including F$_1$ = 103 T, F$_2$ = 127 T and F$_3$ = 160 T, are identified. The
effective masses of the quasiparticles associated with these frequencies are
extracted, namely, m*$_1$ = 0.1 m$_e$, m*$_2$ = 0.1 m$_e$ and m*$_3$ =
0.09m$_e$, where m$_e$ is the free electron mass. From the three-band
Lifshitz-Kosevich fit, the Berry phases accumulated along the cyclotron orbit
of the quasiparticles are 0.06$\pi$, 1.2$\pi$ and 0.74$\pi$ for F$_1$, F$_2$
and F$_3$, respectively. Combined with the ARPES data and the first-principles
calculations, we reveal that F2 and F3 are associated with the two nontrivial
Fermi pockets at the Brillouin zone edge while F1 is associated with the
trivial Fermi pocket at the zone center. In addition, the first-principles
calculations further suggest the existence of Dirac nodal line in the band
structure of SrZnSb$_2$.
