We report low-temperature transport experiments on atomic-size contacts of bismuth that are fabricated using the mechanically controlled break-junction technique at low temperatures. We observe stable contacts with conductance values at fractions of one conductance quantum G(0) = 2e(2)/h, as is expected for systems with long Fermi wavelength. We defer two preferred conductance scales: the lower one is in the order of 0.015 G(0) and can be attributed to single-atom Bi contact, while the higher one amounts to 0.15 G(0), as indicated by the appearance of multiples of this value in the conductance histogram. Rich magneto-transport behaviour with significant changes in the magneto-conductance is found in the whole conductance range. Although for the pristine samples and large contacts with G > 5 G(0), indications for Shubnikov-de Haas oscillations are present, the smallest contacts show pronounced conductance fluctuations that decay rapidly when a magnetic field is applied. Moreover, large variations are observed when a finite bias voltage is applied. These findings are interpreted as the transition from the diffusive to the ballistic and the ultra-quantum regime when lowering the contact size.
