  In some textbooks on quantum mechanics, the description of flux quantization
in a superconductor ring based on the Aharonov-Bohm effect may lead some
readers to a (wrong) conclusion that flux quantization occurs as well for a
long solenoid with the same quantization condition in which the charge of
cooper pair $2e$ is replaced by the charge of one electron $e$. It is shown how
this confusion arises and how can one avoid it.
