The owner of Greece's largest foreign-held bank is making plans to walk away from the bank if the country leaves the euro zone, the latest sign of growing international concern over the future of Europe's currency union. The contingency plan for Crédit Agricole SA, ACA.FR -1.80% France's third-largest publicly traded bank, comes ahead of pivotal elections in Greece Sunday that could set the country on a path to leave the 17-nation currency bloc.