The portion of homes with cell phones but no landlines has grown to 18 percent, led by adults living with unrelated roommates, renters and young people, according to federal figures released Wednesday. An additional 13 percent of households have landlines but get all or nearly all calls on their cells. That means about three in 10 households are essentially reachable only on their wireless phones. The figures, covering the first half of 2008, underscore how consumers have been steadily abandoning traditional landline phones in favor of cells. The 18 percent in cell-only households compares with 16 percent in the second half of 2007, and just 7 percent in the first half of 2005.