A recent result by Fu [Phys. Lett. A 205 (1996) 419] can be found in textbooks. His assertion of the unobservability of an additional arbitrary phase factor is disputed.

Sassoli de Bianchi, M. (1996). Comment on “Relative phase between quantum transmission and reflection amplitudes in one dimension” by Y. Fu. Physics Letters A 219, p. 359. Doi: 10.1016/0375-9601(96)00509-9.