  It was established experimentally that a critical value of the diamagnetic
current, excited by an external magnetic field in a superconducting ring (with
an inductance of about ~ 10^-6 H) with a Nb-Nb clamping point contact having
the Josephson contact properties is a strictly periodic function of field
strength, despite the complex microstructure of the clamping contact. The
reasons of the periodic dependence are discussed on a basis of the interference
model of diamagnetic current and quantized values of the circulating current in
the microinterferometer formed by the clamping contact.
