  ZrTe$_5$ is a newly discovered topological material. Shortly after a single
layer ZrTe$_5$ had been predicted to be a two-dimensional topological
insulator, a handful of experiments have been carried out on bulk ZrTe$_5$
crystals, which however suggest that its bulk form may be a three-dimensional
topological Dirac semimetal. We report the first transport study on ultra thin
ZrTe$_5$ flakes down to 10 nm. A significant modulation of the characteristic
resistivity maximum in the temperature dependence by thickness has been
observed. Remarkably, the metallic behavior, occurring only below about 150 K
in bulk, persists to over 320 K for flakes less than 20 nm thick. Furthermore,
the resistivity maximum can be greatly tuned by ionic gating. Combined with the
Hall resistance, we identify contributions from a semiconducting and a
semimetallic bands. The enhancement of the metallic state in thin flakes are
consequence of shifting of the energy bands. Our results suggest that the band
structure sensitively depends on the film thickness, which may explain the
divergent experimental observations on bulk materials.
