  In a topological semimetal with Dirac or Weyl points, the bulk edge
correspondence principle predicts a gapless edge mode if the essential symmetry
is still preserved at the surface. The detection of such topological surface
state has been considered as the fingerprint prove for crystals with nontrivial
topological bulk band. On the contrary, it has been proposed that even with
symmetry broken at the surface, a new surface band can emerge in nonsymmorphic
topological semimetals. The symmetry reduction at the surface lifts the bulk
band degeneracies, produces an unusual floating surface band with trivial
topology. Here, we report quantum transport probing to ZrSiSe thin flakes and
reveal transport signatures of this new surface state. Remarkably, though
topologically trivial, such a surface band exhibit substantial two dimensional
Shubnikov de Haas quantum oscillations with high mobility, which signifies a
new protection mechanism and may open applications for surface-related devices.
