A USD-4.3-billion (EUR 3.92bn) merger between US sustainable energy company Avangrid Inc (<a href="https://www.nyse.com/quote/XNYS:AGR" target="_blank">NYSE:AGR</a>) and New Mexico-based utilities holding PNM Resources Inc (<a href="https://www.nyse.com/quote/XNYS:PNM" target="_blank">NYSE:PNM</a>) has collapsed after failing to secure final regulatory approval by the deadline of December 31, 2023.