The mobile networks of Verizon and AT&T performed best in April tests throughout lower Fairfield County, but an independent analyst firm said Sprint and T-Mobile appear to have boosted their 4G-LTE coverage locally in the past six months. Bellevue, Wash.-based Rootmetrics conducted some 18,600 tests in early April along 923 miles of roads in the Stamford and Bridgeport areas, as well as in 23 buildings with public access.