  A large decrease is observed in the critical current density of YBCO coated
conductors (CC) and related compounds when a strong perpendicular magnetic
field is applied. While measurements are generally carried out at 77K only,
here we present a magnetic technique permitting to determine the critical
current per unit width of conductor (Icr/w) in a large temperature range. We
report measurements carried out on various CCs that show that, in addition to
the reduction in the critical temperature that can be attributed to the low
value of the irreversibility field near Tc, the field application results in a
large decrease of Icr/w at all temperatures. We ascribe this reduction to the
Josephson behaviour of the twin boundaries included in the YBCO layer.
