  We report on the spontaneous production of fluxons in the presence of a
symmetry-breaking magnetic field for annular Josephson tunnel junctions during
a thermal quench. The dependence on field intensity $B$ of the probability
$\bar{f_1}$ to trap a single defect during the N-S phase transition drastically
depends on the sample circumferences. We show that the data can be understood
in the framework of the Kibble-Zurek picture of spontaneous defect formation
controlled by causal bounds.

Superconductivity