(color online) (a) Some selected Hall resistance measurements for different thickness values of ZrTe₅ films at 1.5 K. The plots display a “suspected” thickness-tuned band transition from electron type to hole type when the thickness is reduced. The Hall anomaly indicates the multi-carrier transport. (b) Some examples of “irregular” thickness-dependent Hall resistance, largely depending on the sample quality. (c) A comparison of the magneto-resistance and Hall resistance of the same sample measured over a one-week interval. The sample was stored in a glove box with an argon atmosphere. Other samples also display a similar decay behavior.