Conduit Street is where Sir John and Lady Middleton reside, along with their family, when they're in town. After Marianne, Elinor, and Mrs. Jennings arrive in London, they are invited to attend a ball at the Middleton's town house. At this ball, Marianne confronts Willoughby and Willoughby denies her affections and any romantic connection. This begins the long process of Marianne's heartbreak and perhaps the beginning of the climax of Marianne's storyline. Willoughby's behavior and denial of their romance crushes Marianne's heart and idyllic and picturesque view of the world. This is the first step of her journey to maturity and embracing sense rather than sensiblity. 

Austen, Jane, and Kathleen Viola James-Cavan. Sense and Sensibility. Broadview Press, 2001.