But if all particle charges are integer multiples of e, solenoids with a flux of 2 π / e have no interference fringes, because the phase factor for any charged particle is exp(2 π i) = 1. Such a solenoid, if thin enough, is quantum-mechanically invisible. If such a solenoid were to carry a flux of 2 π / e, when the flux leaked out from one of its ends it would be indistinguishable from a monopole.