LAUSANNE, Switzerland — Secretary of State John Kerry met with his Iranian counterpart on Wednesday night for what diplomats said could be a series of pivotal sessions as the United States and its negotiating partners sought to secure a preliminary accord to limit Iran’s nuclear program. Kerry met one-on-one with Mohammad Javad Zarif, Iran’s foreign minister, on Wednesday, followed by larger sessions that included the two foreign ministers and a senior European Union diplomat, Helga Schmid. The talks have been snagged by disputes over strict controls on Iran’s uranium enrichment-related research and development and the pace of sanctions relief, especially those imposed by the United Nations, among other issues.