any reversible gate that consumes its inputs and allows all input computations must have no more input bits than output bits, by the pigeonhole principle. for one input bit, there are two possible reversible gates. one of them is not. the other is the identity gate, which maps its input to the output unchanged. for two input bits, the only non-trivial gate is the controlled not gate, which xors the first bit to the second bit and leaves the first bit unchanged.