Apple plans to debut a smaller, cheaper iPad by year-end, said sources. The effort is meant to help maintain dominance of the tablet market as Google and Microsoft prepare competing handheld devices. The new model will have a screen that’s 7 inches to 8 inches diagonally, less than the current 9.7-inch version, said the sources. The new device will probably have a price closer to Google’s Nexus 7 tablet and Amazon’s Kindle Fire, both of which have 7-inch screens and cost $199.