BEIJING -China's exports of rare earth magnets fell in September, reigniting fears that the world's top supplier could wield its dominance over a component key for U.S. defence firms and makers of items from cars to smartphones as leverage in trade talks. In April and May, Beijing squeezed global automakers with export curbs on a range of rare earths items and related magnets, while negotiators faced off over triple-digit U.S. tariffs on goods from the world's second-largest economy.