Boston-based American Tower Corporation (ATC) is reportedly in discussions to buy a controlling stake in Essar Telecom Infrastructure Private Limited (ETIPL) for about $350-$400 million. The two companies have initiated the legal documentation process for transfer of ownership. Barclays Capital is advising Essar on the transaction. ATC and ETIPL had held negotiations twice earlier but the talks failed on account of disagreements over valuation.