All electronic components, even wires, relies on passing electrons from one molecule to another. Transistors involve the use of dissimilar semiconductors, either PNP or NPN, so the smallest possible transistor is three molecules.
However, due to it’s small size, it’s ability to carry current without burning out would be quite limited. As electricity is generated any time you have a conductor, a magnetic field, and relative motion between the two, and as the Earth has a magnetic field, odd are that such a small transistor would be fried the first time it’s moved.
That also assumes that the transistors are hosed in a perfect insulator and isolated from each other. One issues computers have is that transistors close to each other get some signal bleed and crossover, which limits how small we can make usable transistors. At the moment, that lower limit is 10 nanometers for full circuits and 4nm for a single transistor where bleedthrough isn’t an issue, though it’s not cost-effective for them to go below 14nm using current manufacturing techniques.
That said, we may transcend transistors and make quantum computers that rely on other things that perform the same function. Transistors replaced vacuum tubes, so who’s to say something won’t replace transistors?