The Washington region and coastal Easton, Md., are among the nation’s most concentrated enclaves of millionaires, according to a new study of the nation’s wealthiest metropolitan areas. In the Washington area, with its abundance of defense contractors, lobbyists and politicians, 8.5 percent of households are millionaires — ranking fourth nationwide. About 25,700 households accrued more than $5 million of investable assets, the Phoenix Marketing International report shows.