In the tutorial zerothi/ts-tbt-sisl-tutorial/ts_02, it is mentioned that "applying a bias to a bulk system is wrong" and "if you can't immediately figure this out, try and create a longer system by replacing device = elec.tile(3, axis=0) with, say: device = elec.tile(6, axis=0) and redo the calculation for a given bias. then compare the potential profiles."
I know that applying a bias to a pristine system has no physical meaning, but why is it also wrong to apply a bias to a bulk system? From the hint given, it seems that the author simply increased the size of the device model by a factor of two along the current direction. Is this error related to ballistic transport, i.e., when the device size is no longer much smaller than the electron's mean free path? Is my understanding correct?
Here are their potential profiles(the potential in the figures has been amplified by a factor of ten for better visualization). I cannot discern the meaning the author is trying to convey.