  Topological semimetals with symmetry-protected band crossings have emerged as
a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic
symmetries in particular have been shown to play an important role in
protecting the crossings along a line (rather than a point) in momentum space.
Here we report experimental and theoretical evidence for Dirac nodal line
crossings along the Brillouin zone boundaries in PtPb$_4$, arising from the
nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal
lines would remain gapless in the absence of spin-orbit coupling (SOC), the SOC
in this case plays a detrimental role to topology by lifting the band
degeneracy everywhere except at a set of isolated points. Nevertheless, the
nodal line is observed to have a bandwidth much smaller than that found in
density functional theory (DFT). Our findings reveal PtPb$_4$ to be a material
system with narrow crossings approximately protected by non-symmorhpic
crystalline symmetries.
