The soft-drink company will begin testing the 12-ounce container this week at selected food stores in the Columbus, Ga., market. The announcement said plastic cans will contain Coca-Cola, Coca-Cola Classic, Diet Coke and Sprite. It said no other soft drinks in the United States are sold in a plastic can. The plastic used is the same as that in certain soft-drink bottles. The cans will be clear with an aluminum top. Petainer Development Co., in a joint venture with Coca-Cola, is building a factory in Atlanta in which machinery for possible production of plastic cans is being installed. It said the plant is scheduled to be completed in December. An important aspect of the test will be to study collection methods for recycling the plastic cans, the company said. Reverse vending machines that collect, crush and pay consumers for returned plastic cans will be at various Columbus locations during the test period.