> What makes it impossible? possible?
Who says it’s not?
If, by “conductor” you were referring to “a metal wire” the answer is no, it’s impossible to locate one QM entity, like an electron.
It’s difficult to gauge the speed of wind by tracking nitrogen molecules! A single air molecule is able to move around at a random rate of several hundred MPH, regardless of whether the winds are in motion or not. “Wind speed” or “wind velocity” is a general drift that all molecules. To determine wind speed is to let it transport marker particles (smoke and soapy bubbles.) Or, let it deflect a macro object. If not, you’ll need to remove the massive random-walk of the molecule with a single track to only be able to detect the tiny average wind over time.
Electrons behave like this they have bizarre QM effects. Two electrons are not as powerful, and millions of electrons mobile behave like fluids; as a macroscopic object that has the ability to measure velocity and position.
The easiest way to determine the velocity of electrons drifting is to show them in a visible manner. Meiners demonstrates how to do it in his multi-volume compilation of physics-related lecture demos (ref PIRA physics demo 5M10.30) . A transparent crystal of salt is heated, and metal tabs are coiled to the edges. If the crystal is heated with high voltage to the tabs, they will release mobile electrons which create “color centers* that drift in an electric current. IIRC NaCl crystals produces dark gray colors at an negative side. In the course of the electric current it creates an “dark wave” moves across the crystal. When you reverse the drive voltage, it reverses the its direction. We can see the electron cloud and its velocity of drift. Utilize a ruler and stopwatch to gauge the speed.
However, that’s the definition of a “conductor,” not a wire made of metal.